A serious problem with the WECS technology is its wind-energy capturing inefficiency under low wind speed conditions, which limits application scope of the WECS. Since the low-speed wind energy cannot be captured under low wind speed conditions due to the low starting torque, a wind speed of 3-4 m/s is generally regarded as a bottleneck for a horizontal-axis-rotor WECS.
The wind energy below this wind speed cannot be captured effectively to generate the ideal rotating torque for driving a generator, etc., and is thus generally abandoned.
However, in the area easy to be developed, the ratio of the land with the wind speed lower than 4 m/s to the total land is no less than 85% according to statistics, which means that a common WECS is useless in most lands. Therefore, how to capture the wind energy effectively under low wind speed conditions is a common technical problem in the WECS industry.
Explanation will be made below with reference to relevant backgrounds of knowledge and technology. Some terms as well as relevant knowledge and principles are mentioned in many places of the present specification, such as “airfoil”, “lift”, “drag”, “ratio of lift coefficient to drag coefficient”, “angle of attack”, “turning speed”, “tip-speed ratio”, “relationship between tip-speed ratio and wind-energy conversion efficiency”, and “wind-energy capturing efficiency”. These knowledge and principles are well known to those skilled in the fields of aerodynamics and wind power generation, and will thus not be described here in detail. Typical related references include Wind Engineering and Industrial Aerodynamics (compiled by He Dexin, et al, National Defense Industry Press, 1st edition in January 2006), and Wind Energy Handbook (written by Tony Burton, et al, Science and Industry Press, 1st edition in September 2007).
It is known according to general aerodynamic principles about airfoil and lift that for a blade with a given airfoil under given wind speed conditions, the ratio of lift coefficient to drag coefficient and the torque of the blade can be changed synchronizingly by changing the angle of attack of the blade, while the ratio of lift coefficient to drag coefficient and the blade torque are also related to the turning speed of the blade.
When the blade keeps a big angle of attack, it will generate a relatively big rotating torque under low wind speed or low turning speed conditions. Besides, since the drag is in direct proportion to square of the speed, a very big drag torque will be generated under high wind speed or high turning speed conditions, and actually become a damping preventing the rotor from rotating.
When the blade keeps a small angle of attack, it will generate a very big lift torque and a smaller drag torque under high wind speed and high turning speed conditions. However, it can only generate a very small rotating torque under low wind speed conditions, which is limited for staring the rotor.
In summary, it is difficult for a single rotor to both get started easily under low wind speed conditions and possess high efficiency under high turning speed conditions.
Some experiments were made as below: Connecting a high-speed rotor in series with a low-speed rotor, and installing them on a main shaft rigidly, so as to make them be started easily under low wind speed conditions and possess high efficiency under high speed conditions as well. However, this approach proved to be feasible neither theoretically nor practically due to the following reasons: Some measures will naturally be taken in order to get started easily under low wind speed conditions, such as increasing the angle of attack, number of the blades, and the up-wind area; these measures will on one hand surely increase the starting torque, however, on the other hand, the aerodynamic drag produced by the rotor will also increase with accelerated rotation of the rotor; this will not only limit the speed itself, but more importantly impair the turning speed of the main rotor as well, which will thus lower the capturing efficiency under high wind speed and high turning speed conditions, even being not worth the candle. Therefore, it is impractical to superpose the two rotors rigidly in an integrated layout.
The Chinese utility model patent application with publication number No. CN2802116Y has the following design concept: A frame-type multistage WECS for wind power generation is provided; the rotor set is composed of frame-type multistage rotors of different diameters installed coaxially in sequence, with each of the rotors connected with a gear box; the rotation of each of the rotors round the horizontal axis can be converted into the rotation round the vertical axis via a vertical shaft in this gear box, and then transmitted to bottom of a support located on a bearing rotating frame, which is located via a rotary device on a circular stabilizing frame connected with the foundation; a main bearing frame of the bearing rotating frame is provided at one end with a generator, whose shaft is connected with a horizontal shaft pointing to the center of the bearing rotating frame, with the horizontal shaft connected with a downward vertical shaft in the gear box via a pair of tapered gears. This invention, intending to use the wind energy by means of the multistage multiple rotors, has a key disadvantage that it does not operate each of the rotors in a separate controllable mode, because the rotors are different in such properties as power, turning speed and torque. Actually, this invention connects all the rotors mechanically in a closed system and transmits them simultaneously to one main shaft. Therefore, the actual working situation will be as below: The rotor supposed to rotate at a high speed cannot rotate at the supposed high speed due to the low-speed rotor, and cannot therefore output the optimal power; the rotor supposed to rotate at a low speed is driven to rotate at a high speed due to being driven by other rotors, and generates an excessive aerodynamic damping, thus not being able to output the ideal power. In summary, the rotors negatively affect each other, which will prevent the wind energy from being utilized effectively; in addition, the bulky and complicated structure will impair economy, reliability and applicability of this invention. The Chinese Invention patent application with publication No. CN101004168A, with the same inventor as the above-mentioned patent publication No. CN2802116Y, is disclosed mainly for optimizing transmission in the disclosed patent application No. CN2802116Y, however not overcoming other shortcomings mentioned in the present specification.
In summary, all the various disclosed technical approaches cannot attain the purpose of utilizing the wind energy efficiently with one WECS not only under low speed conditions but also under high speed conditions.